I encountered this type of marketing years ago and many are still doing its subtle marketing part of it is content marketing where instead of direct advertising they create a storyline that is relevant to what the audience wants to read of course they mention the platform they are promoting in their storyline.
You are right you have to be innovative to do this and you need engagement, some forums accept this, and some forums don't allow this, and some forums encourage this as long as there are no links in the post.
But it's hard to consider this spam as the content does not revolve around the mentioned product but around the storyline.

Reference to this
The Art of Subtle Marketing: Making Marketing Invisible